Lubricants in commercial use today are prepared from a variety of natural and synthetic basestocks admixed with various additive packages and solvents depending upon their intended application. The basestocks typically include mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone oils, diesters or polyol esters.
Synthetic lubricants provide a valuable alternative to natural lubricants in a wide variety of applications. A preferred synthetic lubricant is neopolyol esters which are formed from the esterification of neopolyols and monocarboxylic acids. Thus, for example, use of neopolyols such as neopentyl glycol, trimethylolethane, trimethylolpropane, monopentaerythritol, technical grade pentaerythritol, dipentaerythritol, tripentaerythritol and the like can be esterified with carboxylic acids ranging from formic acid, acetic acid, propionic acid, up through long chain carboxylic acids both linear and branched. Typically, the acids employed range from C.sub.5 to C.sub.22.
One typical method of production of polyol esters would be to react a neopolyol with a carboxylic acid at elevated temperatures in the presence or absence of an added catalyst. Catalysts such as sulfuric acid, p-toluene sulfonic acid, phosphorous acid, and soluble metal esterification catalysts are conventionally employed.
While the method of production of neopolyol esters as outlined above is well known, the method produces materials with a set of standard properties. For a given combination of neopolyol and acid (or mixtures thereof) there is a set of product properties such as viscosity, viscosity index, molecular weight, pour point, flash point, stability, polarity, and biodegradability which are inherent to the compositions formed by the components in the recipe. To get out of the box of viscosity and other properties imposed by structure, attempts have been made to increase the viscosity of neopolyol esters by means of a second acid, a polybasic acid, in addition to, or instead of, the monocarboxylic acids described above. Thus, employing a polybasic acid such as, e.g., adipic acid, sebacic acid, azelaic acid and/or acid anhydrides such as, succinic, maleic and phthalic anhydride and the like enables one to have the components of a polymeric system when reacted with a neopolyol. By adding a poly- or di-basic acid to the mix, one is able to achieve some degree of cross-linking and/or oligomerization, thereby causing molecular size growth such that the overall viscosity of the system is increased.
High viscosity oils (greater than 100 cSt at 40.degree. C.) are desirable for two reasons: alone, in certain end use applications such as greases, heavy duty engine oils, certain hydraulic fluids and the like, and in blends with lower viscosity oils to achieve a wide viscosity range of products for various iso grades. Complex acid esters like those described above (i.e., esters produced using a polyol, a polybasic acid for cross-linking, and a monobasic acid for end-capping), however, have inherent problems such as high acid numbers and high cost.
An alternative to complex acid esters is complex alcohol esters (esters produced using a polyol, a polybasic acid, and a monohydric linear or branched alcohol for end-capping). With the right ratios of polyol to polybasic acid to monohydric alcohol, complex alcohol esters can be produced which have: reduced cost (approximately half the cost of complex acid esters), high viscosity (greater than 100 cSt at 40.degree. C., good thermal and oxidative stability, good biodegradability, low toxicity, good low temperature properties, excellent lubricity. When blended with lower viscosity oils, a wide range of iso grade products can be produced which meet stringent end-use specifications. The present inventors have discovered that when the amount of linear monohydric alcohol exceed 20% of the total alcohol used, then the pour point is too high, e.g., above -30.degree. C. Furthermore, the present inventors have discovered that the ratio of polybasic acid to polyol is critical in the formation of a complex alcohol ester. That is, if this ratio is too low then a complex alcohol ester contains undesirable amounts of heavies which reduces biodegradability and increases the hydroxyl number of the ester which increases the corrosive nature of the resultant ester which is also undesirable. If, however, the ratio is too high then the resultant complex alcohol ester will have an undesirably low viscosity (reducing its applicability in certain iso grade applications) and poor seal swell characteristics.
Other conventional natural and synthetic esters may each provide one or more of the desired attributes, e.g., high viscosity, good low temperature properties, biodegradability, lubricity, seal compatibility, low toxicity, and good thermal and oxidative stability, but none appears to be able to meet all of the product attributes by themselves. For example, some synthetic esters are capable of meeting the high viscosity property, but fail the biodegradability, low temperature requirements, or low toxicity requirements. Similarly, the natural basestocks such as rapeseed oil are capable of meeting the biodegradability and toxicity properties, but fail to meet the required high viscosity, lubricity, and thermal and oxidative stability properties.
The blended lubricant basestocks according to the present invention comprising a complex alcohol ester and at least one additional natural or synthetic basestock appear to satisfy all of the desired attributes for fully formulated lubricant basestocks by providing the basestock with a unique level of biodegradability in conjunction with effective lubricating properties. They also provide excellent thermal and oxidative stability, good low temperature properties (i.e., low pour points), low toxicity, low volatility, and good seal compatibility.
The complex alcohol esters with low adipate content according to the present invention is formed by using no more than 20% excess alcohol during the reaction step. Furthermore, the present inventors have discovered that these unique complex alcohol esters according to the present invention can also be formed such that they have low metals and acid content by treating the crude reactor product with water at elevated temperatures and pressures greater than one atmosphere. That is, the present inventors have unexpectedly discovered that high temperature hydrolysis can be used to remove a substantial portion of the metal catalyst from the complex alcohol ester reaction product without any significant increase in the total acid number of the resulting product. Low metal and low acid number are important because both can catalyze the hydrolysis of the ester during end-use.
Moreover, the present inventors have also demonstrated that an unexpected, synergistic effect occurs when these complex alcohol esters of the present invention are blended with either a natural or synthetic ester basestock, i.e., the blended basestock unexpectedly exhibits enhanced product attributes versus either the complex alcohol ester or other basestock by itself. Thus, the blended basestocks according to the present invention exhibit the following attributes: excellent lubricity, seal compatibility, biodegradability, low toxicity, good low temperature properties, a wide viscosity range to meet various iso grade needs, good thermal and oxidative stability, and improved engine performance.